Process and a device for the formation of fiberboard

ABSTRACT

A process for the formation of fiberboard where the fiber stock suspension is dewatered on two sides between wires or felts, a top wire being provided in the main dewatering zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 10/053,435, filed Jan. 17, 2002, and U.S. patentapplication Ser. No. 09/402,333 filed Dec. 20, 1999, now abandoned,which was the national phase of International Application No.PCT/EP99/00381 filed Jan. 21, 1999

BACKGROUND OF THE INVENTION

The invention relates to a process and a device for the formation offiberboard. More particularly, the present invention relates to aprocess and device for the formation of fiberboard composed of woodfibers.

Fiberboard made by process and device of the subject invention is of thetype sold as Masonite™ in the United States. Such fiberboard isconstruction grade material used in the manufacture of furniture, andbuilding components such as interior doors.

The classic production process for such fiberboard is the so-called “wetprocess”. This is the preferred ecological production method because thefiberboard can be produced with virtually no chemical bonding agents.Other construction grade fiberboards, such as particle board and MDF(medium-density fiberboard) utilize a dry process. Non-constructiongrade fiberboards, such as cardboard and paste board, may also utilize awet process. However, such fiberboards are manufactured from pulpfibers, not wood fibers, and both the non-construction grade fiberboardsand the pulp fibers from which they are made have mechanical propertieswhich are significantly different from those of construction gradefiberboards and the wood fibers from which they are made. For example,the maximum fiber length of the pulp used for the production ofnon-construction grade fiberboard is approximately 6 mm while themaximum fiber length of the wood fiber used for the production ofconstruction grade fiberboard is approximately 18 mm.

The pulp suspension has a yield (ratio of fibers to total) of more than96% and the wood fiber suspension has a yield of only approximately 80%,as the wood fiber suspension includes a substantial quantity of ligninand other additional components. In addition, paper pulp slurrytypically has a freeness (according to the Canadian Standard FreenessTest) of ≦450 ml CSF, whereas the fiber slurry used in producingconstruction grade fiberboard typically has a freeness of >800 ml CSF.Consequently, the wet process used to manufacture non-construction gradefiberboards is significantly different from the one used to manufactureconstruction grade fiberboard. These differences are sufficient topreclude use of the inventive process for the production ofnon-construction grade fiberboard, and vice-versa.

The apparatus and process for producing non-construction gradefiberboard is similar to that of paper, with the principal differencebetween paper and cardboard being the fibersource, with cardboard beinggenerally formed entirely or partially from secondary (recycled) fibers.Cardboard is normally manufactured in a number of layers (e.g. 3 or 4),where the outermost layers may be manufactured from the virgin fiberswhich are normally used to form paper.

A paper web generally has a basic weight of 30 to 120 gram per m²whereas a cardboard web generally has a basic weight of 200 to 400 gramper m². Construction grade fiberboard has a basic weight in the range of1200 to 8000 g per m² and may have a thickness of one inch or morebefore the final heat press. The dewatering behavior of fiberboard iscompletely different from that of paper and cardboard due to the greaterthickness of the web.

The preparation of the fibers upstream of the machine in both cases istotally different and the properties of the fibers too. Pulp fibers aremore flexible and provide a certain net. Wooden fibers are more stiffand normally require a higher pressure is to be worked. For themanufacture of paper and cardboard, it is essential that the fibers willbe linked together evenly to provide a regular surface. For themanufacture of fiberboard, it is essential to control the thickness andproperly bond the fibers.

The current wet process technology for producing construction gradefiberboard is several decades old. It is ineffective in many processstages and has virtually no means of being regulated. The fibers arereduced in size in a pressurized refiner. Since no (or only very smallquantities of) bonding agents are used, the fibers must developsufficient bonding capacity during the refining process.

The principle of web formation used to date on endless wire machinesdoes have some disadvantages. The present head box technology does notmeet the requirements in terms of formation and calibration of the boardthickness. Furthermore, web formation according to the endless wireprinciple requires a vacuum as driving force for the greater part of thewater to be removed. This means that the energy consumption of a typicalplant is approximately 150 kW. Since the web dewaters to one side, themachine required has to be very long.

SUMMARY OF THE INVENTION

In order to avoid the disadvantage of the chemical bonding agents usedin dry processes, the aim is to improve the wet process used to date.

The invention is thus characterized by the fiber stock suspension beingdewatered on two sides between wires or felts. The energy consumptioncan be greatly reduced as a result because there is no vacuum required.

A further development of the invention is characterized by dilutionwater being added locally in order to regulate the board thicknessacross the web running direction. This is a simple means of evening outthe board thickness without requiring a great deal of design work. Thethickness is then regulated on the basis of the final board thicknessmeasured.

A favorable configuration of the invention is characterized by headboxlips being set locally to regulate the board thickness across the webrunning direction. The board thickness can be set approximately usingthese head box lips, with a more precise board thickness being obtainedin combination with the dilution water.

A favorable further development of the invention is characterized bypre-dewatering taking place in a wedge zone. Due to the rising pressurein the wedge zone, even dewatering to a high consistency can be achievedquickly over a short length and without applying a vacuum, where theboard thickness can be influenced accordingly by using an adjustablewedge zone.

An advantageous further development of the invention is characterized bythe stock being distributed over the working width by means of across-flow distributor and by part of the wood fiber stock suspensionflow being recirculated to the head box. The optimum basis weightcross-profile can be set as a result of this suspension beingrecirculated to the head box.

An advantageous configuration of the invention is characterized by theprocess dewatering to a dry content of more than 40%, preferably morethan 45%. As a result, there is a drop in subsequent pressing time inthe hot press (providing a higher throughput), in the amount of heavilyloaded filtrate produced during hot pressing, and in the amount of steamneeded to evaporate the residual water.

A favorable configuration of the invention is characterized by a toplayer being applied after pre-dewatering, with vacuum extractionpossibly also being provided in the area where the top layer is applied.Thus, it is possible to obtain good fiberboard surface properties byusing small amounts of high-grade fibers without having to alter thecomposition or quality of the rest of the board.

A favorable further development of the invention is characterized byfurther dewatering in a wedge zone after the top layer has been applied.As a result, the entire web, including the top layer, can be dewateredwell.

An advantageous further development of the invention is characterized byseveral points being provided with line pressure, for example two tosix, preferably three to five. A particularly high final dry content canbe achieved as a result.

The invention also refers to a forming device for fiberboard.

This device is characterized by a top wire being provided in the maindewatering zone. With the top wire added, the web can dewater on twosides, virtually halving the dewatering paths, which can alsosubstantially shorten the length of the machine.

An advantageous further development of the invention is characterized bythe top wire forming a wedge zone together with the bottom wire. As aresult, high and controlled pressing force can be applied to the fiberweb, which means that no vacuum is required later for dewateringpurposes.

A favorable configuration of the invention is characterized by the wedgezone being of adjustable design. With this adjustable wedge zone, theboard thickness can be set particularly well.

An advantageous configuration of the invention is characterized by thebottom wire running essentially horizontally as this limits anydisadvantageous effects of gravity.

A favorable further development of the invention is characterized by thewire or felt in the wedge zone being supported by perforated plastic orsteel plates, foil strips or table rolls. Only by using a top wireaccording to the invention is it possible to exploit the advantages ofthe variants mentioned.

A favorable configuration of the invention is characterized by a secondhead box being provided in order to apply a top layer.

An advantageous configuration of the invention is characterized by thewedge zones being suitable for pressure loading at the end, which meansthat the maximum dry content can be obtained after pre-dewatering,regardless of the board thickness to be produced and thus, the webundergoes optimum preparation for the subsequent press zone.

A favorable further development of the invention is characterized byseveral press nips being provided, particularly two to six, preferablybetween three and five, with the press rolls being arranged almostvertically above one another. This achieves a particularly high finaldry content.

A favorable further development of the invention is characterized by thepress rolls in the press nips being suitable for individual pressureloading. As a result, the final dry content and the dewatering curve canbe controlled effectively.

A favorable configuration of the invention is characterized by themachine frame being suitable for cantilevering. This allows the use ofendless woven wires, which provide a longer service life at highpressing forces.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in examples and referring to thedrawings, where

FIG. 1 shows a side view of a state-of-the-art plant,

FIG. 2 shows a horizontal projection of FIG. 1,

FIG. 3 shows a side view of a plant according to the invention,

FIG. 4 shows a section through the line marked IV-IV in FIG. 3,

FIG. 5 a section through the line marked V-V in FIG. 3,

FIG. 6 provides a sectional view of a headbox, and

FIG. 7 shows a sectional view of the pressure loading apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a side view of a endless wire plant 1, with a gravitydewatering zone 2, where the web is guided over rolls 3. Adjoining thisis a zone with vacuum rolls 4, where the greater part of the water isextracted from the web. After this zone, more water is pressed out ofthe web by rolls 5 and 6 mounted in pairs. A typical endless wire plantaccording to the state of the art is approximately 14 m long for athroughput of 180 tonnes/day.

FIG. 2 shows the horizontal projection, illustrating the large number ofrolls 3 required, as well as the vacuum rolls 4 and the press rolls 5and 6. This figure also shows the drive motor 7 with the gearbox 8.

FIG. 3 illustrates a fiberboard plant according to the invention. Itcomprises a head box 22 and an initial dewatering zone 9, where the maindewatering process takes place. This dewatering zone 9 has a bottom wire10, which runs through the entire plant. It also has a top wire 11 sothat the web can dewater in both direction. If no secondary fiber is tobe added, there is only one top wire, which also runs through the entireplant. As a result, the web is dewatered evenly over its entirethickness, which is particularly important in fiberboard production. Asthe water is initially removed from the suspension deposited at thebeginning of dewatering zone 9, the fiber web becomes increasinglyinflexible and rigid.

The first dewatering zone 9 is adjoined by a further dewatering zone 12,where more water is extracted from the web by vacuum boxes 13. Sinceonly a comparatively very thin layer is fed in here, the amount of waterextracted is very small compared with state-of-the-art plants. At theend of this zone 12, a top wire 14 is applied again for furtherdewatering. This wire 14 also runs through the subsequent press zone 15with the rolls 16 mounted in pairs. Here, a final dry content of morethan 40% is achieved, preferably more than 45%, due to the mechanicaldewatering process.

The head box 22 used here can be a cross-flow distributor with adiffuser block and a perforated roll to break up the flocks forming inthe suspension. The bottom wire 10 runs through the entire plant in anessentially horizontal position. A wedge 17 is formed in the firstdewatering zone 9 together with the top wire 11. The wires 10, 11 runover perforated plates 18 here made of plastic or steel. As analternative, foil strips or table rolls can be used. The gap height canbe set at the end 19 of the wedge zone 17 or the wedge zone 17 can bepressure-loaded by the pressure loading apparatus 30. With the pressureloading of the present invention, the gap height is allowed to vary soas to maintain a substantially constant pressure. The constant pressureat the outlet end of the wedge zone 17 produces a consistently dryoutput product in spite of fluctuations in the consistency and ratio ofpulp-to-water of the slurry introduced into the wedge 17. A controller32 monitors the pressure at the outlet end of the wedge zone 17 andadjusts the gap height to maintain a substantially constant pressure.

With reference to FIG. 7, the preferred pressure loading apparatuscomprises a pneumatic or hydraulic tube 34 disposed between a topprofile 36, connected to an immovable upper frame section 38, and abottom profile 40 contacting a frame section 42 connected to the upperplate, respectively. Compressed air or hydraulic fluid is suppliedthrough a connection 44 to move the upper plate 18 and top wire 11relative to the lower plate 18 and bottom wire 10 and thereby controlthe gap height.

A roll 20, driving against the top wire 11, forms the end of the wedgezone. At the dewatering zone 12, a further headbox 23 can be providedfor a top layer. In order to dewater the top layer, a further top wire14 is provided. Dewatering is assisted by extraction using vacuum(through boxes 13). The top wire 14 and the bottom wire 10 form afurther wedge zone 21, which is also adjustable and can be designed forpressure-loading at the end of the zone if necessary. In order toincrease the final dry content, the press zone 15 contains two to six,preferably three to five, press nips, i.e. pairs of rolls pressedagainst one another. The present example shows four such roll pairs 16which form press nips. A plant of this type has an overall length ofapproximately 11.5 m for a throughput of approximately 320 tonnes/day,i.e. although production is increased by approximately 80%, only some80% of the length required in a state-of-the part plant is needed here.This provides a specific output of approximately 220% compared withstate-of-the-art plants.

FIG. 4 shows a section through the line marked IV-IV in FIG. 3. In thissection viewed against the web running direction, the bottom wedge plate18, a vacuum box 13 and the roll 20 at the end of the wedge zone 17 areall clearly shown. The so-called front side is marked FS and the rearside, where all of the drives and other leads and lines are located, ismarked TS.

FIG. 5 shows a section through the same point as FIG. 4, however viewedin the web running direction. Here, the rear and front sides arereversed compared with FIG. 4. In addition to the vacuum box 13 and thewedge plate 18, this illustration also shows the second headbox 23 forthe top layer. The suspension to the headbox 23 is fed through aconnection pipe 24 coming from the rear side. The water extracted isremoved from the vacuum box 13 through a pipe 25.

FIG. 6 shows the headbox 22 in detail. The suspension is fed into across-flow distributor 26 in the headbox 22 under pressure from a stockfeeding pump (not shown). The headbox 22 acts as a pressure boundarysuch that the pressurized suspension flows through a diffuser block 27,a perforated roll 28, and out of an outlet gap 29. Any defects in thedistribution of the wood fiber in the suspension are corrected beforethe suspension exits the headbox 22, the perforated roller 28 breakingdown any knots or flocks that have formed in the suspension and evenlydistributing the wood fiber in the liquid of the suspension at theoutlet gap 29. Rollers in the forming zone downstream of the headbox ofprior art devices acted to break down knots and flocks in the woodfiber. However, the liquid component of the suspension is required foreffective distribution of the wood fiber and since the material wasalready partially dewatered, any particles of wood fiber of such brokenknots and flocks could not be effectively redistributed, effectivelyleaving the defect intact.

The wood fiber suspension is discharged from the headbox 22 into thewedge zone 17 formed by the top and bottom wires 11, 10. It should beappreciated that the use of top and bottom wires 11, 10 allows the webto be dewatered on both sides simultaneously. This leads to better andmore even physical properties of the fiberboard produced by theinventive system. It should also be appreciated that it would not beappropriate to use top and bottom wires in the production of relativelythin materials, such as paper and cardboard, since the belts wouldproduce unacceptable marks in the surface of the material and since thelow basic weight of the material precludes dewatering by pressing thematerial between opposed belts.

The invention is not limited to the examples described. It would also bepossible to combine the individual dewatering zones in different ways,depending on the given requirements.

1. Device for forming wood fiber board from a wood fiber stocksuspension has a basic weight of 1200 to 8000 g/m², the devicecomprising a first headbox; a main dewatering zone having a first topwire or felt and a bottom wire or felt, the first top wire and thebottom wire forming a first wedge zone having an inlet end and an outletend; and apparatus for pressure loading the outlet end of the firstwedge zone.
 2. Device according to claim 1 wherein the first wedge zoneis adjustable.
 3. Device according to claim 1 wherein the bottom wireruns essentially horizontally.
 4. Device according to claim 1 furthercomprising support means for supporting the wire or felt in the firstwedge zone said support means comprising perforated plastic, steelplates, foil strips or table rolls.
 5. Device according to claim 1further comprising a second headbox for applying a top layer.
 6. Deviceaccording to claim 5 further comprising a second dewatering zone havinga second top wire or felt and the bottom wire or felt forming a secondwedge zone.
 7. Device according to claim 6 wherein the second wedge zonehas an inlet end and an outlet end, the second wedge zone being pressureloaded at the outlet end.
 8. Device according to claim 1 furthercomprising several press nips, each of the press nips including opposedpress rolls.
 9. Device according to claim 8 wherein the press rolls arearranged substantially vertically above one another.
 10. Deviceaccording to claim 8 wherein the press rolls in the press nips aresuitable for individual pressure loading.
 11. Device according to claim1 wherein the machine frame is suitable for cantilevering.
 12. Deviceaccording to claim 1 further including two to six press nips.
 13. Deviceaccording to claim 1 further including three to five press nips. 14.Device according to claim 1 further including a controller forcontrolling the apparatus for pressure loading.
 15. Device according toclaim 14 wherein the controller and apparatus for pressure loadingmaintain a substantially constant pressure load at the outlet end of thefirst wedge zone.
 16. Device according to claim 14 wherein the apparatusfor pressure loading is a pneumatic or hydraulic tube.